Stress sensor

ABSTRACT

A stress sensor enduring against long-term use is provided. Accordingly, a substrate ( 4 ), which is used as both a sensor part ( 1 ) and a supporting part ( 2 ), functions as a stress sensor, in which the sensor part ( 1 ) has means for deforming a part thereof in response to a given stress and strain gauges ( 5 ) having a function for varying electric properties in response to the deformation, and in which the deforming part has a stress dispersing means ( 10 ).

TECHNICAL FIELD

[0001] The present invention relates to stress sensors which can beused, for example, for a pointing device for personal computers, or amultifunctional and multidirectional switch for various electronicdevices.

BACKGROUND ART

[0002] A stress sensor has been disclosed in Japanese Unexamined PatentApplication Publication No. 2000-267803, in which strain gauges 22formed by film formation are disposed on a surface of a substrate 20, apost 30 is bonded to another surface of the substrate 20, and thedirection and magnitude of a stress applied to the post 30 can begrasped from variation in property of the strain gauges 22 resultingfrom the application of the stress.

[0003] As shown in FIGS. 14(a) and 14(b), the structure comprises: fourresistor elements functioning as the strain gauges 22 provided withtrimming grooves 21, which are disposed on two lines, being along asurface of the substrate 20 and perpendicularly intersecting each otherat the center of the surface of the substrate 20, at substantially thesame distance from the center mentioned above; and the post 30 having asquare bottom surface bonded so that the center of the bottom surfacethereof substantially coincides with the center of the substrate 20 andthat each side of an outline 30 b of the post bottom surface faces eachof the resistor elements 22. In addition, the trimming grooves 21 areformed at two positions of each of the resistor elements 22, the twopositions being along each side of the outline 30 b of the post bottomsurface and provided at the rear side of the substrate 20 correspondingthereto.

[0004] In addition, the movement of the stress sensor is shown in FIG.13(a) in which a stress is applied to the post 30 in an X direction(that is, an optional lateral direction) and in FIG. 13(b) in which astress is applied to the post 30 in a Z direction (that is, a downwarddirection).

[0005] In the movement of the stress sensor described above, in bothcases in which a stress is applied to the post 30 in an X axis or a Yaxis direction as shown in FIG. 13(a) and in which a stress is appliedto the post 30 in a Z axis direction as shown in FIG. 13(b), solder 32fixed by a circuit board 31 fixes end portions of the substrate 20, andthe stress primarily warps positions of the substrate 20 correspondingto the individual sides of the outline 30 b of the post bottom surface.In addition, the structure is formed in which by the stress describedabove, the strain gauges 22 which are the resistor elements disposed atthe positions described above are elongated or contracted.

[0006] However, in the case of the structure of the above conventionalstress sensor, there has been a problem in that the sensitivity (output)in response to the stress applied to the post 30 is low. It has beenbelieved that the reason for this is that since the stress applied tothe post has not been designed to be concentrated on the strain gaugesor the design thereof has not been made sufficiently, the stress islikely to be disperses widely over the substrate 20, and as a result,the applied stress has not been effectively used.

[0007] Accordingly, a first object that the present invention aims toachieve is to provide a stress sensor having high sensitivity.

[0008] In addition, as shown in FIGS. 13(a) and (b), when the operationis performed many times to elongate or contract the resistor elements22, the elongation or contraction may exceeds the region of elasticdeformation in some cases to cause plastic deformation. Due to thisplastic deformation, the output resistance from the resistor element 22in response to subsequent stress application becomes incorrect. Thereason for this is that since the plastic deformation is a deformationin which reversibility is lost, the original shape cannot be recoveredeven when the stress is removed, and a stress resulting from the plasticdeformation of the substrate 20 is always applied to the resistorelements disposed on the substrate 20 as described above.

[0009] In particular, as shown in FIG. 14(b), when the trimming grooves21 of the resistor elements 22 are formed along the outline 30 b of thepost bottom surface, it may be naturally expected that the movement toelongate or contract the resistor elements 22 may cause movement to openand close the trimming grooves 21 as shown in FIGS. 13(a) and (b). Inthe case described above, it is not too much to say that the plasticdeformation of the resistor elements 22 is facilitated. The reason forthis is that the trimming groove 21 portions are liable to beplastic-deformed as compared to the other portions. This is because ofvery large energy which is applied to a resistor forming the resistorelement 22 when the trimming grooves 21 are formed.

[0010] For example, in the case of laser trimming, the resistor ispartly and instantaneously heated to a high temperature, so that thepart of the resistor is removed by evaporation thereof. Since thisremoval process is performed concomitant with large and very rapidchange in temperature, of course, cracks may be generated around theperiphery of the trimming groove 21 in some cases. The cracks thusgenerated may widely extend by the movement to open and close thetrimming groove 4. As a result, it is expected that the plasticdeformation may occur from the cracks as starting sites.

[0011] Other trimming methods also partly excavate or damage theresistor forming the resistor element 22 as is the case of the lasertrimming. When cracks are generated in the resistor by the reason asdescribed above, a factor serving to embrittle the resistor isadditionally generated. As the trimming methods other than lasertrimming, for example, sand blasting may be mentioned.

[0012] Accordingly, a second object that the present invention aims toachieve is to provide a stress sensor which can achieve the first objectand which can maintain the accuracy of output resistance by suppressingthe plastic deformation of resistors which are used as strain gauges andform resistor elements provided with trimming grooves.

DISCLOSURE OF INVENTION

[0013] In order to achieve the first object described above, a stresssensor having a first structure of the present invention is a stresssensor in which strain gauges 8 are disposed on a surface of a substrate1, a post 6 is disposed on one of surfaces of the substrate 1, and thedirection and magnitude of a stress applied to the post 6 can be graspedfrom variation in property of the strain gauges 8 resulting from theapplication of the stress. In the stress sensor described above, thestrain gauges 8 are disposed on and the post 6 is bonded to orintegrated with the same surface of the substrate 1. As the straingauges 8, for example, resistor elements 2 each formed of a thick or athin film, or piezoelectric elements formed of PZT (lead zirconatetitanate) may be preferably used.

[0014] In addition, in order to achieve the first object describedabove, a stress sensor having a second structure of the presentinvention is a stress sensor in which the strain gauges 8 are disposedon a surface of the substrate 1, the post 6 bonded to one of surfaces ofthe substrate 1, and the direction and magnitude of a stress applied tothe post 6 can be grasped from variation in property of the straingauges 8 resulting from the application of the stress. In the stresssensor described above, a post 6 bottom surface and a part or the entirearea of each of the strain gauges 8 overlap each other without thesubstrate 1 provided therebetween.

[0015] In general, stress sensors each comprise a control unit in whichthe electrical properties described above are, for example, detected andcomputed, thereby functioning as a stress sensor. However, in thisspecification, for convenience, a portion excluding the control unitdescribed above is referred to as a “stress sensor”.

[0016] In addition, “the post 6 is bonded to a surface of the substrate1” indicates the state in which the post 6 and the substrate 1 aredifferent members and are fixed together with an adhesive or the like.In addition, “the post 6 is integrated with a surface of the substrate1” indicates the state in which the post 6 and the substrate 1 are, forexample, integrally formed. In this specification, when the “outline ofthe post bottom surface” is present in the latter case, the expressionindicates a portion corresponding to that represented by the “outline ofthe post bottom surface” in the former case.

[0017] By the first structure described above, a stress sensor havinghigh sensitivity to a stress applied to a Z direction can be provided ascompared to that in the past. The reason for this will be described. Forexample, when the post 6 and the resistor elements 2 are mounted on thesame surface of the substrate 1 as shown in FIG. 1(a), and a stress isapplied to the post 6 in the Z direction, although the warpage of thesubstrate 1 obtained by the stress application is equal, between thewarpage of a resistor element 2 disposed on the surface of the substrate1 which is warped to form a concave shape and the warpage of a resistorelement 2 disposed on the surface of the substrate 1 which is warped toform a convex shape, the warpage of the resistor element 2 disposed onthe surface of the substrate 1 which is warped to form a convex shape islarger because of the difference in curvature radius of the surface.That is, the variation (output) in resistance of the resistor elements 2caused by the stress application in the Z direction can be increased.This tendency is enhanced with increase in thickness of the substrate 1.Accordingly, the preferable thickness of the substrate 1 is 0.3 to 1.2mm. When the thickness of the substrate 1 is less than 0.3 mm, itbecomes difficult to obtain a significant difference in curvatureradius. In addition, when the thickness of the substrate 1 is more than1.2 mm, although depending on a material for the substrate 1, warping ofthe substrate 1 becomes unlikely to occur by the stress applied thereto,and adversely, it is believed that the variation (output) in resistanceof the resistor elements 2, which is caused by the stress applied in theZ direction, becomes unlikely to be increased.

[0018] In addition, by the reason approximately equivalent to thatcapable of increasing the output in the Z direction, it is naturallyunderstood that output in X and Y directions can also be increased.

[0019] In the stress sensor of the present invention, when some functionis created (added) by using the stress application to the post 6 in adownward direction (z direction) as described above, multifunctionalitycan be enhanced. For example, when the stress sensor of the presentinvention is used as a pointing device of a computer, a so-calledmouse-clicking function may be served by the stress application in thedownward direction described above. In addition, for example, when thestress sensor of the present invention is used as a multidirectionalswitch of a compact mobile device such as a so-called mobile phone,stress application in the downward direction for a predetermined timemay correspond to the instruction on ON and OFF operation of a powersource of the mobile device.

[0020] Another advantage of the first structure in which the straingauges 8 are disposed on and the post 6 is bonded to or integrated withthe same surface of the substrate 1 is that the stress sensor of thepresent invention can be manufactured by performing mounting operationonly on one side surface of the substrate 1 and that easiermanufacturing can be performed thereby. The mounting operation mentionedabove includes, for example, screen printing of the surface of thesubstrate 1 with conductors 5, resistors 3, and the like forming theresistor elements 2, and bonding of the post 6 to the surface of thesubstrate 1 with an adhesive or the like. On the other hand, in the casein which the mounting is performed on the two surfaces of the substrate1, while the mounting is performed on one surface of the substrate 1, aposition at which the other surface of the substrate 1 is placed must becontrolled under strict conditions in terms of cleanness, softness, andthe like. From this point of view, when the mounting is performed on thesame surface of the substrate 1, the strict conditions as describedabove is not required.

[0021] Another advantage obtained in the case in which the strain gauges8 are disposed on and the post 6 is bonded to or integrated with thesame surface of the substrate 1 is that the alignment of the straingauges 8 and the post 6 can be easily performed. The positionalrelationship between the strain gauges 8 and the post 6 is asignificantly important factor that determining the performance of thestress sensor. For example, in FIG. 2, when the position of the post 6is largely deviated, the stress applied to the post 6 is propagated in adifferent manner to each of the strain gauges 8. The reason for this isthat the positions on the outline 7 of the post bottom surface, at whichthe strain gauges 8 are warped, are deviated. In the case in which thepost 6 and the resistor elements 2 are mounted on different surfaces ofthe substrate 1, when one surface of the substrate 1 is visuallyobserved, the other surface of the substrate 1 cannot be observed.Hence, it has been difficult to understand the positional relationshipbetween the post 6 and the resistor elements 2, and as a result, thepositional deviation therebetween is relatively liable to occur.However, when both the post 6 and the strain gauges 8 are mounted on thesame surface of the substrate 1, the relative positional relationshipbetween the post 6 and the strain gauges 8 is very easily grasped, andhence the positional deviation described above is unlikely to occur. Inaddition, visual inspection can be easily performed when an elementwhich is disposed once at a deviated position is removed.

[0022] According to the second structure in which the post 6 bottomsurface and a part or the entire area of each of the strain gauges 8overlap each other without the substrate 1 provided therebetween, astress sensor having an improved sensitivity to the stress applied inthe X and Y directions, in addition to the Z direction, can be provided.The reason for this is that the stress applied to the post 6 almostdirectly stimulates the resistor elements 2 without through thesubstrate 1. As the result of this stimulation, the strain gauges 8 arecompressed. For example, in FIG. 1(b), one example is shown. In thisfigure, the structure is shown in which the strain gauges 8 (resistorelements 2) are disposed on the top surface of the substrate 1, and apart of each of the strain gauges 8 (resistors 3 portion) is disposed soas to overlap a post 6 bottom portion. By the stimulation describedabove, the resistors 3 are partly compressed, and as a result, theresistance thereof is increased.

[0023] In addition, of course, the second structure described above hasthe two advantages of the first structure described above. In addition,when a conventional stress sensor outputs a stress applied in the Zdirection, it is necessary to provide a gap at a surface of thesubstrate 1, different from that on which the post 6 is disposed, sothat the substrate 1 is warped in the Z direction; however, according tothe second structure shown in FIG. 1(b), an advantage can be obtained inthat the gap described above is not always necessary. However, it ispreferable when the gap described above is provided since thesensitivity to the stress applied in the Z direction can be furtherimproved.

[0024] An important function of the second structure is a functioncapable of grasping the direction and magnitude of an applied stressfrom variation in property of the strain gauges 8 caused by a pressureapplied thereto and the removal thereof resulting from the applicationof the stress.

[0025] When the structure has the important function described above, itis not necessary to limit the positions at which the strain gauges 8 aredisposed to the surface of the substrate 1. For example, in thestructure shown in FIG. 1(b), the disposition may be made on the bottomsurface of the post 6. In this case, it is believed that an advantage inthat the stress sensor can be miniaturized on the whole is obtained.However, since a manufacturing method in which the disposition is madeon a flat substrate 1 is easily performed as compared to a manufacturingmethod in which the strain gauges 8 are disposed on the bottom surfaceof the post 6, in recent years, it has been believed that the advantageof the second structure is more significant.

[0026] In addition, in order to achieve the first object of the presentinvention, a stress sensor having a fourth structure of the presentinvention is a stress sensor in which the direction and magnitude of astress applied to the post 6 can be grasped from variation in resistanceof the resistor elements 2, which are not provided with trimming grooves4, caused by stimulation applied thereto resulting from the applicationof the stress. In the stress sensor described above, the stimulation isprimarily applied to resistor 3 regions in which a current density ishigh.

[0027] In the fourth structure described above, the substrate 1 is notan essential factor. That is, the resistor elements 2 may be formed on asurface of the substrate 1 or may be formed, for example, on sidesurfaces of the post 6. That is, the structure may be used in which theresistor elements 2 are stimulated caused by the application of thestress to the post 6.

[0028] That is, the stimulation described above is, for example,elongation and contraction of the strain gauges 8 disposed on thesubstrate 1 caused by warping of the side surface of the post 6 or thesubstrate 1, shown in FIG. 1(a); a pressure applied to the strain gauges8 and the removal thereof by the post 6 bottom surface without throughthe substrate 1, shown in FIG. 1(b); or elongation and contraction ofthe strain gauges 8 disposed at the side surfaces of the post 6 by usingthe warping of the post 6 itself, which are not shown in the figure.

[0029] In the resistor element 2, since a resistor 3 region having anarrow current pass is a region in which a current density is high, whenthis region is primarily stimulated, the variation in resistance, thatis, the output of the stress sensor, can be increased as compared to thecase in which another region is stimulated. Accordingly, by using thestress sensor having the fourth structure described above, a stresssensor capable of efficiently converting the stress applied to the post6 into the variation in resistance can be provided, and hence the firstobject can be achieved. In addition, since a first stress sensor is notprovided with the trimming grooves 4, of course, plastic deformation isunlikely to occur even when the resistors 3 are stimulated, and hence itmay be said that the second object is achieved.

[0030] The state in which “the stimulation is primarily applied to theresistor 3 region having a narrowed current pass” means the state inwhich a maximum part of the distribution of the stress applied to theresistor 3 in the resistor 3 region is present in the resistor 3 regionhaving the narrowed current pass.

[0031] In addition, in the resistor element 2, for positively formingthe resistor 3 region having a narrowed current pass, for example, whena resistor patterning is performed, for example, by screen printing forforming a thick-film resistor, means for forming a resistor partlyhaving a narrow width, when it is viewed from above the pattern, iseffective. In addition, for example, means is also effective in whichprotruding convex portions are provided on parts of a surface of thesubstrate on which thick-film resistors are to be provided, a resistorpaste used for screen printing is applied to flow from tops of theconvex portions to a lower side, followed by treatment of stopping theflow of the resistor paste (firing, curing, or the like), so that thinresistor portions are formed at the convex portions. In addition, theformer and the latter means may be used in combination.

[0032] A stress sensor having a fifth structure of the presentinvention, which achieve the second object, is a stress sensor in whichthe direction and magnitude of a stress applied to the post 6 can begrasped from variation in resistance of the resistor elements 2, whichare provided with the trimming grooves 4, caused by stimulation appliedto the resistor elements 2 resulting from the application of the stress.In the stress sensor described above, the stimulation described abovedoes not substantially open and close the trimming grooves 4 and isprimarily applied to the resistor 3 regions in which a current densityis high.

[0033] The reason the fifth structure can achieve the first object isthe same reason as that for a fourth stress sensor which can achieve thefirst object. In addition, the reason a fifth stress sensor can achievethe second object is that the stimulation does not substantially openand close the trimming grooves 4, and that plastic deformation of theresistors 3, starting from cracks around the trimming grooves 4, isunlikely to occur. In order to form a stress sensor having the structurein which the stimulation does not substantially open and close thetrimming grooves 4, for example, in a stress sensor which moves in amanner as shown in FIG. 13, means for forming the positionalrelationship in which the outline 7 of the post bottom surface and thetrimming grooves 4 perpendicularly intersect each other, as brieflyshown in FIG. 2, may be mentioned. The reason the trimming grooves 4 arenot substantially opened and closed by the structure described above isthat the direction in which the resistor element 2 is stimulated(elongated and contracted) by warping of the substrate 1 approximatelycoincides with the direction in which the trimming grooves 4 are formed.Accordingly, even when the stress sensor is used many times, theresistors 3 are not liable to be plastic-deformed, and hence it may besaid that the second object can be achieved.

[0034] In addition, as shown in FIG. 3, to locate the trimming grooves 4only on a surface of the substrate 1 inside the outline 7 of the postbottom surface is effective for substantially suppressing the open andclose of the trimming grooves 4. The reason for this is that theposition of the substrate 1 to which the post 6 bottom surface is bondedis not substantially warped by the movement shown in FIG. 13, and thatthe stress is unlikely to be propagated to the resistors 3 through theposition described above. By the same reason as described above, asshown in FIG. 2, to locate the trimming grooves 4 only on a surface ofthe substrate 1 outside the outline 7 of the post bottom surface iseffective for substantially suppressing the open and close of thetrimming grooves 4. The reason for this is that the part of thesubstrate 1 along the outline of the post bottom surface is most warped.

[0035] In addition, as shown in FIG. 2, to locate the trimming grooves 4only on a surface of the substrate 1 outside the outline 7 of the postbottom surface is also effective for substantially suppressing the openand close of the trimming grooves 4. In a stress sensor having thestructure in which the post 6 and the strain gauges 8 are disposed onsurfaces of the substrate 1 opposite to each other, when trimminggrooves 4 are present in resistor regions located only outside or insidethe outline 7 of the post bottom surface, the trimming grooves 4described above are formed at positions apart from those correspondingto the outline 7 of the post bottom surface, at which the largestdeformation of the substrate 1 and the strain gauges 8 (resistorelements 2) occurs by the stress applied to the post 6. Hence, thestress applied to the trimming grooves 4 can be suppressed as small aspossible, and as a result, the structure described above has anincreased contribution to the achievement of the second object.

[0036] In the fifth structure shown in FIGS. 2 and 3, the resistor 3regions in which the current passes are narrowed by the trimming grooves4 are located outside the positions of the substrate 1 corresponding tothe outline 7 of the post bottom surface at which the substrate 1 iswarped by the stress applied to the post 6. Accordingly, the regionsdescribed above become regions in which the current density is highest,and since the regions are primarily stimulated (elongated andcontracted), a stress sensor capable of efficiently converting thestress applied to the post 6 to the variation in resistance can beprovided so as to contribute the achievement of the first object.

[0037] In the fourth and the fifth structures described above, forexample, as shown in FIGS. 1(a) and (b), it is preferable that theresistor elements 2 be disposed on the same surface of the substrate 1and that the post 6 be bonded to or integrated with the surface of thesubstrate 1. The reason for this, that is, the advantage is the same asthe advantage obtained by the first to third stress sensors.

[0038] In addition, in order to achieve the first object, a stresssensor having a sixth structure is a stress sensor in which the resistorelements 2, provided with no trimming grooves 4, are disposed on asurface of the substrate 1, and in which the direction and magnitude ofa stress applied to the post 6 can be grasped from variation inresistance of the resistor elements 2 caused by a pressure appliedbetween the post 6 bottom surface and the surface of the substrate 1 andthe removal thereof resulting from the application of the stress withoutthrough the substrate 1. In the stress sensor described above, thepressure is primarily applied to resistor regions in which a currentdensity is high.

[0039] The sixth structure described above clearly shows that, inaddition to the stimulation caused by the elongation and contraction ofthe resistor elements 2 resulting from the warping of the substrate 1,the pressure applied to the resistors 3 and the removal thereof are alsoeffective in the present invention. The mechanism of a sixth stresssensor of the present invention for achieving the first object isapproximately equivalent to that of the first and second stress sensors.In the case described above, as a member involved in the application ofthe pressure, the post 6, that is, a member to which a stress isapplied, is advantageously used since the loss of the stress can bedecreased, and accurate direction and magnitude of the stress can bepropagated. In this case, the largest pressure is applied to resistor 3regions which are in contact with or correspond to the outline 7 of postbottom surface.

[0040] In addition, it is expected that the sixth structure can achievethe second object of the present invention. The reason for this is thatit is considered that, by the stimulation (pressure or removal thereof)to the resistors 3, the trimming grooves 4 may not be substantiallyopened and closed. In addition, in a stress sensor having the structurein which the trimming grooves 4 are not directly pressed and theresistor 3 portions other than the trimming grooves 4 are onlystimulated, the structure described above, of course, achieves thesecond object of the present invention.

[0041] In the sixth structure, since the outline 7 of the post bottomsurface, which applies the largest pressure to the resistors 3, islocated in the resistor 3 regions in which the current pass is narrowed(for example, resistor 3 regions in which the current pass is narrowedby the trimming grooves 4), the structure described above contributes tothe achievement of the first object of the present invention.

[0042] In addition, in order to achieve the first and the secondobjects, a stress sensor having a seventh structure of the presentinvention is a stress sensor provided with one of the first to the sixthstructures. In the stress sensor described above, the four strain gauges8 formed of the resistor elements 2 are disposes on two lines,perpendicularly intersecting each other at a center of a sensoreffective region on a surface of the substrate 1, at substantially thesame distance from the intersecting point; the post 6 is bonded orintegrated so that the center of the sensor effective region on thesurface of the substrate 1 substantially coincides with the center ofthe post 6 bottom surface; and the direction and magnitude of a stressapplied to the post 6 can be grasped from variation in resistance causedby elongation and contraction of the resistor elements 2, or by apressure applied thereto and the removal thereof, resulting from thestress applied to the post 6. In this case, the “center” of the above“center of the sensor effective region” and “center of the post 6 bottomsurface” does not strictly mean the center point but it includes a shiftfrom the center point, in which the stress sensor effectively functions.

[0043] As the description has made clear, the structure shown in FIG. 2is the first structure and may also be the seventh structure. Inaddition, the state in which the structure shown in FIG. 2 is providedand, as shown in FIG. 1(b), the post 6 bottom surface overlaps a part orthe entire surface of each of the strain gauges 8 without the substrateprovided therebetween is also the seventh structure.

[0044] In addition, in order to achieve the first and the secondobjects, a stress sensor having an eighth structure of the presentinvention is a stress sensor in which one of the first to the seventhstructures is provided, a post bottom portion 12 has projecting portions15, and by a stress applied to the post 6, the projecting portions 15 ofthe post bottom portion 12 primarily stimulate the strain gauges 8 orthe resistor elements 2.

[0045] By the eighth structure described above, a stress sensor havinghigher sensitivity to the stress applied to the post 6 can be provided,and the reason for that is as follows. The sensitivity described abovecan be improved by increasing an amount of elongation, contraction, orcompression of the strain gauges 8 such as the resistor elements 2.Accordingly, as is the eighth structure described above, by providingthe projecting portions 15 at the post bottom portion 12, the stressapplied to the post 6 can be concentrated on the projecting portions 15.When the projecting portions 15 stimulate the strain gauges 8, theconcentrated stress is propagated to the strain gauges 8, and as aresult, the amount of elongation, contraction, or compression isincreased as compared to that in the past.

[0046] In the past, the post bottom portion 12 also had the projectingportions 15. For example, in the post 30 shown in FIG. 14, the outershape of the bottom surface is square, and the angular portions thereofcorrespond to the projecting portions 15 of the post bottom portion 12.However, the positions at which the angular portions are disposed do notcorrespond to the strain gauges 8, and as a result, the projectingportions 15 do not stimulate the strain gauges 8. Accordingly, most ofthe stress concentrated on the angular portions is not propagated to thestrain gauges 8, and consequently, the amount of elongation,contraction, or compression of the strain gauges 8 is not increased.

[0047] In the eighth structure described above, in the case in which thepost 6 and the strain gauges 8 are disposed on different surfaces of thesubstrate 1, when the substrate 1 is too thick, dispersion of the stressbecomes excessively high, and as a result, the stress becomes unlikelyto be propagated to the strain gauges 8. In addition, when the thicknessof the substrate 1 is too thin, by repeated stress concentration, theshape of the substrate 1 becomes unlikely to be recovered. That is, thesubstrate 1 may be plastic-deformed by exceeding the region of elasticdeformation in some cases. In consideration of the cases describedabove, the preferable thickness of the substrate 1 is in the range offrom 0.5 to 0.8 mm. Although varying depending on a material for thesubstrate 1, the thickness is approximately in the range describedabove.

[0048] A particular example of the eighth structure and a preferablestructure based thereon is the structure in which the outer shape of thepost 6 bottom surface is a polygon as shown in FIG. 8, and in which theangular portions of the polygon serve as the projecting portions 15. Thenumber of the angular portions of the polygon is preferably equivalentto that of the strain gauges 8. The reason for this is that when apolygon having angular potions larger than the number of the straingauges 8 is used, the stress applied to the post 6 is likely to beconcentrated on potions other than the strain gauges 8 (that is, thestress is likely to be dispersed), and as a result, the stress thusapplied cannot be efficiently propagated to the strain gauges 8. In thestructure shown in FIG. 8, as described above, the number of the straingauges 8 is four, and the polygon is square.

[0049] In addition, in the eighth structure and the group of preferablestructures based on these described above, it is preferable that theouter shape of the substrate 1 have at least one pair of sides which areparallel to each other, that a top portion of the post 6 be in the formof a polygonal pole having at least one pair of side surfaces which areparallel to each other, and that said pair of sides and said pair ofside surfaces be parallel to each other. Those described above arerealized by the post 6 shown in FIG. 8. That is, the top portion of thepost 6 is a tall square pole and has a pair of facing side surfaceswhich are parallel to each other. In addition, the outer shape of thesubstrate 1 is square and has a pair of facing sides which are parallelto each other. In addition, when viewed from above, the sides formingthe outer shape of the substrate 1 and the sides forming the outer shapeof the top portion of the post 6, which are located at respectivepositions to the above sides, are all parallel to each other. Hence, thestructure is formed in which said pair of sides and said pair of sidesurfaces are parallel to each other. By using the structure as describedabove, workability of bonding the post 6 to the substrate 1 may beimproved in some cases. The reason for this is that a holding directionof a known mounting device which holds and moves a workpiece (the topportion of the post 6 when the post 6 is held) is not changed, and thatthe movement described above is performed only in optional x and ydirections, that is, a movement in a θ direction, i.e., a rotationalmovement, is not performed. When the substrate 1 and the post 6 aremounted by a known mounting device while aligned, the workability issignificantly improved in view of simplification. In the case describedabove, due to limited functions of a know mounting device, the structurein which said pair of sides of the substrate 1 and said pair of sidesurfaces of the post 6 are parallel to each other is required.

[0050] In addition, in the eighth structure and the group of preferablestructures based on these described above, it is preferable that theprojecting portion 15 have a round shape. The reason for this is thatthe first object of the present invention can be achieved even when thestress concentrated at the strain gauges 8 is dispersed to some extent,and it is believed that the round shape will not cause serious problems.In addition, when the stress concentration is dispersed to some extentby forming the round shape, as described above, the plastic deformationof the substrate 1 described above and the plastic deformation of thestrain gauges 8 can also be suppressed. It is believed that the effectof the round shape is particularly advantageous in the state in whichthe post 6 bottom surface overlaps-a part or the entire area of each ofthe strain gauges 8 without the substrate 1 provided therebetween. Thereason for this is that the plastic deformation of the strain gauges 8can be suppressed, the strain gauges 8 generally formed of a materialwhich is softer than that for the substrate 1 and is likely to beplastic-deformed as compared thereto.

[0051] In all the structures of the present invention described above, aprotection film directly covering at least the strain gauges 8 is morepreferably formed. The protection film is preferably formed of amaterial softer than that for the substrate 1 and the strain gauges 8.As the material described above, in general, a silicone-based resinmaterial, a rubber material, or the like may be mentioned. In thestructure in which the post 6 bottom surface almost directly stimulatesthe strain gauges 8, such as the second structure described above, thesoft material has an effect of dispersing the stress in a predeterminedrange (in general, approximately in the region of common strain gauges8) to a proper extent when the stimulation is performed. Hence, thestress described above is not only transmitted to limited parts of thestrain gauges 8 but is also sufficiently transmitted to the entire areasof the strain gauges 8, and as a result, the plastic deformation of thestrain gauges 8 can be suppressed. In addition, in the structure shownin FIG. 1(a) of the present invention, the soft material has an effectof suppressing the decrease in adhesion between the substrate 1 and thestrain gauges 8, which is caused by repeated warping of the straingauges 8 following the warping of the substrate 1.

[0052] Among the soft materials, a silicone-based resin material is notliable to be degraded by repeated deformation, can maintain a highadhesive strength between the substrate 1 and the resistor elements 2,and can reliably protect the resistor elements 2 for a long period oftime, and hence the silicone-based resin material is preferably used.

[0053] In all the structures of the present invention described above,the post 6 is preferably composed of a metal, a ceramic, a resin, or afiber-reinforced resin. The advantage obtained when a metal, such asiron or high carbon steel, or a ceramic is used as a material for thepost 6 is that a stress applied thereto can be accurately propagatedbecause of the rigidity of those described above. In addition, a firstadvantage obtained when a resin or a fiber-reinforced resin is used as amaterial for the post 6 is that when the production thereof isperformed, less energy is consumed. For example, a temperature formolding and curing a resin or a fiber-reinforced resin is very low ascompared to a sintering temperature for a ceramic and a castingtemperature for a metal. A second advantage is superior moldability tothat of ceramic and metal. For example, when a post 6 having acomplicated shape is formed, cracking may occur in ceramic during amolding or sintering step and in metal during a casting step in somecases. The reason for this is that, during cooling, the rigid materialcannot follow the volume contraction thereof caused by a temperaturechange from a very high temperature to room temperature. On thecontrary, when a resin of a fiber-reinforced resin is used, since amelting temperature of a resin is very low as compared to the sinteringtemperature and the casting temperature described above, the volumecontraction during cooling is small, and in addition, the rigidity of aresin is low as compared to that of a metal or a ceramic, it is saidthat the problem described above may not occur at all.

[0054] This post 6 may be used when the stress sensor of the presentinvention is applied to a pointing device for a personal computer or amultifunctional and multidirectional switch for various electronicdevices such as a mobile phone, in particular, a compact mobileelectronic device. In the case in which the stress sensor of the presentinvention is used as the multifunctional and multidirectional switchdescribed above, in order to enable an operator to recognize by feelinga direction in which a stress is to be applied, it is preferable that across-sectional shape of a side surface of the post 6 be polygonal sothat each instruction can be transmitted to an electronic device byapplying a stress perpendicularly to each flat surface on the side ofthe post 6. When the complication of forming the post 6 having thecross-sectional polygonal shape described above is taken intoconsideration, the post 6 is preferably formed of a resin or afiber-reinforced resin as described above.

[0055] In addition, as a material when a resin is used, in particular,poly(vinyl terephthalate) (PVT) is preferably used. Since PVT hassuperior rigidity among resin materials, an advantage is obtained inthat a stress applied can be relatively accurately propagated. Inaddition, since the heat stability is also superior, even when the useenvironment is at a temperature slightly higher than room temperature,an advantage in that the rigidity described above is maintained can alsobe obtained.

[0056] In addition, in the first to the eighth structures and preferablestructures based thereon, it is preferable that the substrate 1 beprimarily composed of a resin, a metal covered with a non-conductivematerial on the surface thereof, or a ceramic. As the material primarilycomposed of a resin, for example, a phenolic resin itself, or afiber-reinforced resin such as a molded body made of a glass fiberfilled epoxy resin may be mentioned. As the metal covered with anon-conductive material on the surface thereof, an iron or an aluminumplate coated with a polyethylene resin may be mentioned. As the ceramicmentioned above, for example, alumina may be used. In addition toflexibility to be warped to some extent, the substrate 1 must also haveboth rigidity and elasticity so as to be able to recover its own shapewhen a stress repeatedly applied thereto is removed, and all thematerials described above by way of example can satisfy the aboverequirements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0057]FIG. 1(a) is a stress sensor of the present invention in whichresistor elements and a post are provided on one surface of a substrate1, and in addition, (b) shows the state in which the bottom surface of apost overlaps the resistor elements.

[0058]FIG. 2 is a schematic view showing the state in which straingauges are disposed on a substrate forming the stress sensor of thepresent invention.

[0059]FIG. 3 is a view showing the arrangement of trimming grooves ofresistor elements forming the stress sensor of the present invention.

[0060]FIG. 4 is a front view of a large alumina substrate used formanufacturing the stress sensors of the present invention.

[0061]FIG. 5 is a view showing an example of a general input-outputstate of electrical signals when the stress sensor of the presentinvention is used.

[0062]FIG. 6 is a front view showing a large alumina substrate used formanufacturing the stress sensors of the present invention.

[0063]FIG. 7 includes views showing an example of a process formanufacturing the stress sensor of the present invention.

[0064]FIG. 8 includes views showing examples of a top view and a sideview of an example of the stress sensor of the present invention.

[0065]FIG. 9 includes views showing various shapes of trimming grooveswhich are optional constituent elements of the stress sensor of thepresent invention.

[0066]FIG. 10 includes views showing various shapes of trimming grooveswhich are optional constituent elements of the stress sensor of thepresent invention.

[0067]FIG. 11 includes views showing various shapes of post projectingportions which are optional constituent elements of the stress sensor ofthe present invention.

[0068]FIG. 12 includes views showing an example of a process formanufacturing the stress sensor of the present invention.

[0069]FIG. 13 includes views showing an example of a conventional stresssensor, in which the movement thereof is shown.

[0070]FIG. 14 includes views showing an example of the structure of aconventional stress sensor.

[0071] Reference numerals in the figures indicate as follows, 1 . . .substrate, 2 . . . resistor element, 3 . . . resistor, 4 . . . trimminggroove, 5 . . . conductor, 6 . . . post, 7 . . . outline of post bottomsurface, 8 . . . strain gauge, 9 . . . terminal, 10 . . . dividinggroove, 11 . . . large alumina substrate, 12 . . . post bottom portion,13 . . . protection film, 15 . . . projecting portion, 16 . . . hole, 17. . . through-hole, 18 . . . gap-forming member, 20 . . . substrate, 21. . . trimming groove, 22 . . . resistor element, 23 . . . postoperation unit, 24 . . . conductor, 30 . . . post, 30 b . . . outline ofpost bottom surface, 31 . . . circuit board, and 32 . . . solder.

BEST MODE FOR CARRYING OUT THE INVENTION

[0072] Hereinafter, a first embodiment of the present invention will bedescribed.

[0073] As shown in FIG. 4, a large alumina substrate 11 is prepared inwhich a plurality of units each having an octagonal outline is providedat positions defined by many dividing grooves 10 extending inlongitudinal and lateral directions. The substrate described above isprovided with reinforcing members. The reinforcing members are to beformed consequently by the layout of the dividing grooves 10 and holes16, and, of course, have the same thickness as that of a substrate 1 fora stress sensor. This reinforcing member has a first function ofpreventing warping generated when a great number of the triangular holes16 are formed by punching or the like in manufacturing the large aluminasubstrate 11. In addition, the reinforcing member has a second functionof protecting a squeegee from being deformed and damaged when thesqueegee is pressed onto a surface of the large alumina substrate 11 inscreen printing described later, which is performed several times.

[0074] Onto the surface of the large alumina substrate 11, an Ag-Pdbased conductive paste is applied by screen printing, and by firing,conductors 5 shown in FIG. 2 are obtained. Next, a ruthenium oxide-basedresistor paste is applied by screen printing to form resistor elements 2in combination with the conductors 5 shown in FIG. 2, and by firing,resistors 3 are formed.

[0075] Next, laser trimming is performed so that each of the fourresistors 3 has a predetermined resistance, thereby forming trimminggrooves 4. In this case, as shown in FIG. 2, the trimming grooves 4 areeach formed in a resistor 3 portion at an end portion side of thesubstrate 1. When the trimming grooves 4 are each formed in the portiondescribed above, a powdered conductor (in this case, a powder of amaterial forming the resistor 3) scattered in trimming is fixed aroundthe individual end portions of the substrate 1 and the dividing grooves10. Accordingly, the probability can be decreased as small as possiblein that resistor elements 2 adjacent to each other on the same substrate1 are connected to each other by the presence of the powder so that thefunction as the stress sensor cannot be fully obtained.

[0076] Subsequently, a silicone-based resin is further screen-printed soas to cover all the four resistor elements 2 including the resistors 3,thereby forming a protection film (not shown in the figure) through acuring step. In this step, the thickness of the protection film is setto 10 to 30 μm so as to protect the resistor elements 2 from plasticdeformation resulting form an excessive application of a stress to theresistor elements 2 and to prevent an excessive decrease in sensitivityin response to the application of the stress to a post 6. In order tosuppress the variation of the sensitivity described above as small aspossible, the thickness of the protection film is preferably set to 15to 20 μm. Accordingly, a mother substrate of the substrates 1 eachhaving the layout of the resistor elements 2 shown in FIG. 2 isobtained.

[0077] As shown in FIG. 2, the post 6, formed of poly(vinylterephthalate) (PVT) and having a square bottom surface, is fixed atapproximately the center of the substrate 1 with an epoxy-based adhesiveso that the bottom surface is brought into contact with the same surfaceas that of the substrate 1 on which the resistor elements 2 are disposedand overlaps the resistor 3 portions of the resistor elements 2. In thisstep, the overlapping areas are formed so as to be approximately equalto each other.

[0078] Next, a force is applied to the large alumina substrate 11 so asto open the dividing grooves 10 for separating (dividing) the individualstress sensors from each other, thereby obtaining the stress sensors ofthe present invention. In this case, the conductor 5 portions located atthe outer ends of the substrate 1 serve as terminals 9 through whichelectrical signals are sent to and from a control unit. The stresssensor of the present invention thus obtained is a stress sensorcomprising: resistor elements 2, current flow directions of which aresubstantially parallel to the respective sides of an outline 7 of thepost bottom surface, are provided with the trimming grooves 4 formed inthe side opposite to that facing the outline 7 of the post bottomsurface and are disposed on two lines, being along a surface of thesubstrate 1 and perpendicularly intersecting each other at the centerthereof, at substantially the same distance from the center of thesubstrate 1; and the post 6 bonded so that the center of the bottomsurface thereof having a square outline coincides with the center of thesubstrate 1 and that the individual sides of the outline 7 of the postbottom surface face the respective resistor elements 2. In this stresssensor described above, the direction and magnitude of a stress appliedto the post 6 can be grasped from variation in resistance of theresistor elements 2 caused by elongation or contraction thereofresulting from the application of the stress, and the post 6 bottomsurface and the strain gauges 8 (resistor elements 2) overlap each otherwithout the substrate 1 provided therebetween.

[0079] The opposite surface of the substrate 1, from that to which thepost 6 is bonded, of the stress sensor thus obtained is mounted so as toface a printed circuit board. In this printed circuit board, wiring isprovided for the control unit, for example, for detecting and computingelectrical properties (variation in resistance) of the stress sensor andis electrically connected to the terminals 9 with solder. In this case,an epoxy resin adhesive is screen-printed on positions of the printedcircuit board corresponding to the end portions of the substrate 1 ofthe stress sensor, and the stress sensor is placed on and fixed to thesurface of the printed circuit board. Hence, the cured adhesivedescribed above is disposed instead of solder 32 shown in FIG. 13, andas shown in the same figure, warping of the substrate 1 can be realizedwhen the stress is applied to the post 6 in an optional direction of X,Y, and Z directions. In addition, this warpable region corresponds tothe “sensor effective region” on the surface of the substrate 1described above.

[0080]FIG. 5 shows a general input-output state of electrical signals ofthe stress sensor according to the present invention. The four resistorelements 2 form a bridge circuit. Between voltage application terminals(Vcc)—(GND) of this bridge circuit, a predetermined voltage is applied.In addition, the resistor elements 2 and a Y terminal (Yout), providedat the left side in the figure, form a stress sensor in the Y axisdirection, and in addition, the resistor elements 2 and an X terminal(Xout), provided at the right side in the figure, form a stress sensorin the X axis direction.

[0081] In the first embodiment, the step of fixing the posts 6 to thesubstrates 1 is performed before the large alumina substrate 11 isdivided; however, the step described above may be performed after thedivision described above. However, after the individual substrates 1 areseparated from each other, since the handling thereof becomes difficult,the step described above may cause problems in some cases. Accordingly,as is the first embodiment, the step of fixing the posts 6 to thesubstrates 1 is preferably performed before the large alumina substrate11 is divided.

[0082] Hereinafter, a second embodiment of the present invention will bedescribed.

[0083] As shown in FIG. 6, the large alumina substrate 11 is prepared,in which units having a square outline are each defined by the dividinggrooves 10 extending in longitudinal and lateral directions and crossinga great number of thorough-holes 17.

[0084] Onto each bottom surface shown in FIG. 7(g) of the substrates 1of the large alumina substrate 11 surface, an Ag-Pd based conductivepaste is first applied by screen printing, and by firing, the conductors5 (FIG. 7(h)) are obtained. Next, an Ag-Pd based conductive paste isapplied so as to form a pattern shown in FIG. 7(b), and by firing, theconductors 5 are obtained. The screen printing for forming theconductors 5 described above is performed by so-called through-holeprinting, and as shown in a side view in FIG. 8, through the conductors5 (terminal 9 described later) on the side walls of through-holes 17formed at the side surfaces of the substrate 1, the conductors 5 on thetop surface and the bottom surface of the substrate 1 are connected toeach other.

[0085] Next, a ruthenium oxide-based resistor paste is screen-printed soas to form the resistor elements 2 in combination with the conductors 5shown in FIG. 7, and by firing, the resistors 3 are obtained (FIG.7(c)). Next, laser trimming is performed for individual four resistors 3so as to have a predetermined resistance, thereby forming the trimminggrooves 4 (FIG. 7(d)).

[0086] Subsequently, a silicone-based resin is further screen-printed soas to cover all the four resistor elements 2 including the resistors 3,and through a curing step, a protection film 13 is obtained (FIG. 7(e)).In this step, the thickness of the protection film is set to 10 to 30μm. In order to suppress the variation in sensitivity of the resistorelements 2 to the stress applied to the post 6, it is preferable thatthe thickness of the protection film be uniform in the range of fromapproximately 15 to 20 μm. In this step, the positions of the conductors5 and the resistors 3, which have been previously formed, can be graspedas concaves and convexes of the protection film 13 for improving thesensitivity to the stress applied to the Z direction described above.Accordingly, the effect of the grasping the relative positionalrelationship between the post 6 and the resistor elements 2, describedabove, is not lost. Furthermore, onto the bottom surface of thesubstrate 1, an epoxy resin paste is applied by screen printing so as toserve as a gap-forming member 18 (described later) having a thickness ofapproximately 50 μm (FIG. 7(i)).

[0087] In addition, as shown in FIG. 7(f), approximately to the centerof each substrate 1, the post 6 having the square bottom surface, madeof a molded poly(butylene terephthalate) (PBT) part, is fixed with anepoxy-based adhesive by using a known mounting device so that the bottomsurface of the post 6 is brought into contact with the same surface ofthe substrate 1 as that on which the resistor elements 2 are disposed tooverlap the resistor 3 portion of each of the resistor elements 2, andthat projecting portions 15 (angular portions at the four corners of apost 6 bottom portion) are located to face the respective the resistor 3regions in each of which a current pass is narrowed by the trimminggroove 4 formed in the resistor 3. In this case, the overlapping areasare formed to be approximately equivalent to each other. According tothis structure, the projecting portion 15 stimulates the resistor 3region in which the current pass is narrowed. Hence, a mother substrateof the stress sensors of the present invention is obtained.

[0088] Next, a force is applied to the large alumina substrate 11 so asto open the dividing grooves 10 for separating (dividing) individualstress sensor units from each other, thereby obtaining the stresssensors of the present invention. The surface of the substrate 1,opposite to that to which the post 6 is bonded, of the stress sensorthus obtained is mounted so as to face a printed circuit board. In thisprinted circuit board, wiring is provided for the control unit, forexample, for detecting and computing electrical properties (variation inresistance) of the stress sensor and is electrically connected to thestress sensor through the terminals and fixed thereto with solder. Inthis case, the gap-forming member 18 described above is used instead ofthe solder 32 shown in FIG. 7, and as shown in the same figure, warpingof the substrate 1 can be realized when the stress is applied to thepost 6 in an optional direction of X, Y, and Z directions. In addition,this warpable region corresponds to the “sensor effective region” on thesurface of the substrate 1 described above. When this warpable region isshown in the figure, the area in which the protection film 13 isprovided as shown in FIG. 7(e) is approximately corresponding thereto.The reason for this is believed that since the side wall surfaces of thethrough-holes 17 are fixed to the above-mentioned printed circuit boardwith solder through a reflow step or the like, the four corners of thesubstrate 1 may not serves as the warpable region. However, the centerof the warpable region (the center of the sensor effective region) is apoint at which the diagonal lines extending from the four corners of thesubstrate 1 perpendicularly intersect each other. The center of the post6 bottom surface shown in FIG. 7(f) is disposed at a position thatapproximately coincides with the center of the warpable region.

[0089] The general input-output state of electrical signals of thestress sensor shown in FIG. 7 can be made equivalent to that shown inFIG. 5.

[0090] In the second embodiment, the step of fixing the posts 6 to thesubstrates 1 is performed before the large alumina substrate 11 isdivided; however, the step described above may be performed after thedivision described above. However, after the individual substrates 1 areseparated from each other, since the handling thereof becomes difficult,the step described above may cause problems in some cases. Accordingly,as is the second embodiment, the step of fixing the posts 6 to thesubstrates 1 is preferably performed before the large alumina substrate11 is divided.

[0091] In the second embodiment, the substrate 1 having a square outlineis used. The advantage thereof is easy manufacturing of the stresssensor. That is, when a large alumina substrate 11 containing a greatnumber of octagonal substrates 1 is manufactured, a step must beperformed beforehand of forming the relatively large holes 16 in asquare shape or the like in the substrate described above by punching orthe like. In addition, in the punching step described above or thescreen printing in the second embodiment, the large alumina substrate 11may be warped in some cases. As a result, subsequent handling of thesubstrate 1 or the properties of the stress sensor may bedisadvantageously influenced in some cases. Accordingly, it has beenbelieved that the substrate 1 having a square outline is suitably used.

[0092] Hereinafter, refereeing to FIG. 9, the structures of the trimminggrooves 4, each formed in a resistor 3 region which is present onlyoutside the outline 7 of the post bottom surface, obtained through thesteps of the first or the second embodiment of the present inventionwill be described.

[0093]FIG. 9(a) shows the structure in which, in accordance with thepositional relationship between the resistors 3 and the post 6 in FIG.2, so-called L cut is used as a method for forming the trimming groove4. This is also an example the structure in which the trimming groove 4is primarily formed so as to be substantially parallel to the directionin which the resistor element 2 is elongated or contracted, and issubordinately formed so as to be substantially perpendicular to thedirection described above. The trimming grooves 4 shown in FIG. 1 andthe like each have a linear shape, and the trimming method therefor iscalled single cut. When the trimming groove 4 is formed form the outsideof the resistor 3 by laser trimming or the like, a large residual stressmay be generated in the position which is last irradiated with laser insome cases, resulting in the generation of cracks. In addition, thecrack thus formed tends to extend along the direction in which thetrimming groove 4 is formed. As a result, even if the crack as describedabove is generated, an effect of suppressing adverse influences causedby the generation of the crack as small as possible can be obtained byallowing the crack to extend in the direction approximately parallel tothat in which current flows. Since the effect described above can beobtained, the L-cut is preferably selected as the trimming method.

[0094]FIG. 9(b) shows the structure in which, in accordance with thepositional relationship between the resistors 3 and the post 6 in FIG.2, the trimming groove 4 is formed by so-called hook cut. This is anexample of the structure in which the trimming groove 4 is primarilyformed so as to be substantially parallel to the direction in which theresistor element 2 is elongated or contracted, and is subordinatelyformed so as to be substantially in a direction except that parallel tothe direction described above. The effect of this hook cut isapproximately equivalent to that of the L cut described above.

[0095]FIG. 9(c) shows the structure in which, in accordance with thepositional relationship between the resistors 3 and the post 6 in FIG.2, a plurality of the trimming grooves 4 is formed by the single cutdescribed above. In this trimming method described above, when a firsttrimming groove 4 is formed, very fast formation of the trimming groove4 is performed until a predetermined value or a predetermined ratio isobtained with respect to a final desired resistance. Next, when a secondtrimming groove 4 is formed, slow formation of the trimming groove 4 isperformed so as to obtain the final desired resistance. Since the speedfor forming the second trimming groove 4 is slow, an effect of improvingthe resistance accuracy can be obtained. In addition, since the speedfor forming the first trimming groove 4 is increased, it may be saidthat the effect of improving the resistance accuracy can be obtainedwithout extremely increasing the total tome for the trimming operation.

[0096]FIG. 9(d) shows the structure in which, in accordance with thepositional relationship between the resistors 3 and the post 6 in FIG.2, the width of the trimming groove 4 is increased. As a method forforming the trimming groove 4 described above, for example, lasertrimming may be mentioned, in which grooves for removing resistor (eachcorresponding to each of the trimming grooves 4 shown in FIGS. 9(a) to(c)) are formed, for example, so as to be approximately parallel to acurrent flow direction in the resistor 3 and be adjacent to each otherin the width direction of the groove, and so that a material forming theresistor 3 does not substantially remain between the grooves forremoving resistor formed adjacent to each other. By the method describedabove, a trimming groove 4 having a width equivalent to the length ofthe groove for removing resistor is formed. A very small amount of thematerial forming the resistor 3 may remain as long as being placed in adistribution state which causes no influence on the resistance of theresistor element 2. By using this trimming method, significantlysuperior resistance accuracy can be obtained. The reason for this isthat the variation in resistance of the resistor element 2 per unitlength of the groove for removing the resistor 3 can be extremelydecreased. In addition, by the structure described above, the resistanceafter the trimming operation is stabilized. The reason for this is thatthe resistance tends to be stabilized with increase in width of thetrimming groove 4. The stability of the resistance means the stabilitywith respect to ambient environment such as an ambient temperature.

[0097]FIG. 9(e) shows the structure in which, in accordance with thepositional relationship between the resistors 3 and the post 6 in FIG.2, the second trimming groove 4 shown in FIG. 9(c) is replaced with thetrimming groove 4 shown in FIG. 9(d). The method for forming thetrimming groove 4 shown in FIG. 9(d) takes a long period of time;however, by the structure described above, without spending an extremelylong period of time, a resistor element 2 having significantly superiorresistance accuracy can be obtained.

[0098]FIG. 9(f) shows the structure in which the current flow directionin the resistor element 2 and the direction of the elongation andcontraction thereof are parallel to each other, and in which thetrimming grooves 4 are formed in the resistor 3 at the side opposite tothat facing the outline 7 of the post bottom surface. The structuredescribed above includes the case in which with respect to the half ofthe current flow length of the resistor 3, the trimming grooves 4 areformed at the side opposite to that facing the outline 7 of the postbottom surface. In the same figure, each groove is formed in each of theleft and the right sides of the resistor 3. The reason for this is thatwhen the trimming grooves 4 are formed at only one of the left and theright sides, the resistor element 2 at the side at which the trimminggrooves 4 are formed exhibits the decrease in sensitivity to a stress.When the balance in sensitivity of the resistor element 2 is distorted,as described above, for example, inconvenience may occur in some cases,in which with respect to a stress applied to the post 6 in a specificdirection, the result (information) of the stress application may have aslight deviation in terms of output direction. Accordingly, inapplications in which very strict directional accuracy is required, thisinconvenience may become a problem; however, for example, when thestress sensor is used as a pointing device of a computer, since it isbelieved that the very strict directional accuracy is not required, thestress sensor described above can be used without any problems. Inaddition, when the entire size of the resistor element 2 is decreasedand the length of the trimming groove 4 may be extremely decreased, thedistortion of the balance described above can be decreased to a levelenough to be ignored, and the problems may not occur at all.

[0099] The structures shown in FIGS. 9(a) to (f) are the case in whichthe post 6 and the resistor elements 2 are disposed on the differentsurfaces of the substrate 1; however, in the case in which the post 6and the resistor elements 2 are disposed at the same surface of thesubstrate 1 as shown in FIGS. 1(a) and (b), it is naturally understoodthat each of them is the first or the second embodiment of the presentinvention.

[0100] Hereinafter, refereeing to FIG. 10, the structures of thetrimming grooves 4, each formed in a resistor 3 region which is presentonly inside the outline 7 of the post bottom surface, obtained throughthe steps of the first or the second embodiment of the present inventionwill be described.

[0101]FIG. 10(a) shows the structure in which, in accordance with thepositional relationship between the resistors 3 and the post 6 in FIG.3, the so-called L cut is used as a method for forming the trimminggroove 4. The reason the L cut is selected is the same as described inFIG. 9(a). In the L-cut, since a groove following a first groove, whichis perpendicular thereto, is generally short, and the substrate insidethe outline 7 of the post bottom surface is very unlikely to be deformed(primarily, warping), the trimming groove is not substantially openedand closed by the stress applied to the post 6.

[0102]FIG. 10(b) shows the structure in which, in accordance with thepositional relationship between the resistors 3 and the post 6 in FIG.3, the trimming groove 4 is formed by so-called hook cut. This is anexample of the structure in which the trimming groove 4 is primarilyformed so as to be substantially parallel to the direction in which theresistor element 2 is elongated or contracted, and is subordinatelyformed so as to be substantially in a direction except that parallel tothe direction described above. The hook cut described above has aneffect approximately equivalent to that of the L cut described above.

[0103]FIG. 10(c) shows the structure in which, in accordance with thepositional relationship between the resistors 3 and the post 6 in FIG.3, a plurality of the trimming grooves 4 is formed by the single cutdescribed above. By using the trimming method described above, theeffect is obtained which improves the resistance accuracy withoutextremely increasing the total time for the trimming operation, and thereason for this is the same as that described in FIG. 9(c).

[0104]FIG. 10(d) shows the structure in which, in accordance with thepositional relationship between the resistors 3 and the post 6 in FIG.3, the width of the trimming groove 4 is increased. The reason thesignificantly superior resistance accuracy can be obtained by formingthe trimming grooves 4 described above using the method for forming thetrimming grooves 4 described above is equivalent to that described inFIG. 9(d).

[0105]FIG. 10(e) shows the structure in which, in accordance with thepositional relationship between the resistors 3 and the post 6 in FIG.3, the second trimming groove 4 shown in FIG. 10(c) is replaced with thetrimming groove 4 shown in FIG. 10(d). The method for forming thetrimming groove 4 shown in FIG. 10(d) takes a long period of time;however, by the structure described above, without spending an extremelylong period of time, a resistor element 2 having significantly superiorresistance accuracy can be obtained.

[0106] It is naturally understood that these structures shown in FIGS.10(a) to (e) may be applied to the case in which the post 6 and theresistor elements 2 are disposed at the different surfaces of thesubstrate 1 and may also be applied to the case in which the post 6 andthe resistor elements 2 are disposed on the same surface of thesubstrate 1 as shown in FIGS. 1(a) and (b).

[0107] Hereinafter, with reference to FIG. 11, an example of a stresssensor formed through the steps of the first or the second embodiment ofthe present invention will be described, in which the post bottomportion 12 has the projecting portions 15, and the projecting portions15 primarily stimulate the strain gauges 8. In FIG. 11, membersnecessary for descriptions are only shown, and the other members (forexample, the conductors 5 shown in FIG. 10(a)) are omitted.

[0108]FIG. 11(a) shows an example in which the projecting portion 15 hasa round shape when the stress sensor is viewed from above. In addition,FIG. 11(b) shows an example in which the projecting portion 15 has around shape when the stress sensor is viewed from the side. It isnaturally understood that the effect obtained when the projectingportions 15 is rounded described above can be achieved in both casesshown in FIG. 11(a) and FIG. 11(b). In addition, the structuresdescribed above may be used in combination. In addition, as shown inFIG. 11(c), the post 6 and the strain gauges 8 may be disposed ondifferent surfaces of the substrate 1, and the top surfaces and/or theside surfaces of the projecting portions 15 may be rounded.

[0109]FIG. 11(d) shows the structure in which although the projectingportions 15 and the strain gauges 8 are provided so as to correspond toeach other, they are provided at a certain distance from each other. Thestructure described above is expected to be effective, for example, whena material having very superior sensitivity (exhibiting a largevariation in property even by a small elongation, contraction, orcompression) is used for the strain gauges 8. This structure may beapplied to both cases in which the post 6 and the strain gauges 8 aredisposed on different surfaces of the substrate 1 and in which the post6 and the strain gauges 8 are disposed on the same surface of thesubstrate 1.

[0110]FIG. 11(e) shows the structure in which the post bottom portion 12is circular when viewed from above, and in which the post 6 bottomsurface overlaps the strain gauges 8 without the substrate 1 providedtherebetween. In this case, the entire periphery of the post bottomportion serves as the projecting portion 15. In the case in which thepost 6 bottom surface overlaps the strain gauges 8 without the substrate1 provided therebetween, the projecting portion 15 having the structuredescribed above may be able to concentrate a stress applied to the post6 thereon and to propagate it to the strain gauges 8 so as to obtain aneffect in which an amount of elongation, contraction, or compressionthereof is large as compared to that in the past. In a general stresssensor, when a stress is propagated to the strain gauges 8 from the post6 bottom surface through the substrate 1, although the size of the post6 bottom portion may have some influence, the projecting portion 15,which is the entire periphery of the post bottom portion, does notstimulate the strain gauges 8 with a highly concentrated stress. In thiscase, it is believed that a cylinder is preferably formed as the post 6in which the top and the bottom portions thereof have the same diameter,since the post 6 is most easily manufactured.

[0111]FIG. 11(f) shows an example in which the projecting portion 15shown in FIG. 11(e) has a round shape when the stress sensor is viewedfrom the side.

[0112]FIG. 11(g) shows an example of the case shown in FIG. 11(c), inwhich the projecting portions 15 are provided on the post 6 bottomsurface. These projecting portions 15 may be formed when the post 6 isformed or may be subsequently formed by bonding an optional material(including a material different from that for the post) at optionalpositions of a smooth bottom surface of the post 6. This structure mayalso be applied when the post 6 and the strain gauges 8 are disposed onthe same surface of the substrate 1.

[0113]FIG. 12 includes views showing a process for manufacturing thestress sensor according to the second embodiment of the presentinvention in order to compare the process shown in FIG. 7, the stresssensor described above having the structure in which the trimminggrooves 4 are formed only inside the outline 7 of the post bottomsurface as shown in FIG. 3. The difference form FIG. 7 is only thepositions of the trimming grooves 4 shown in FIG. 7(d) and FIG. 12(d).

[0114] In the stress sensor having the structure shown in FIG. 12, whena stress is applied to the post 6, the post 6 bottom surface presses thetrimming grooves 4, and in view of this point described above, thestress sensor described above is different from that shown in FIG. 7.

[0115] Since the stress sensor shown in FIG. 12 has the projectingportion 15 at the four corners of the post bottom portion 12, a stressapplied to the post 6 is propagated to the resistor 3 portion, whichcorresponds to the most outside end of the post bottom portion 12 and tothe projecting portion 15, and presses the portion described above. Theportion described above is a position in which the trimming groove 4 isnot formed, and as a result, the trimming groove 4 is not primarilypressed, and the resistor 3 portion, in which a current pass thereof isnarrowed by the trimming groove 4, is primarily pressed. As a result,the first object can be achieved.

[0116] In addition, by the stress application described above, damage ofthe resistors 3 in the vicinity of the trimming grooves 4, which iscaused by an excessive stress applied thereto, is not generated, and thetrimming grooves 4 are not also opened and closed, whereby the secondobject can also be achieved.

INDUSTRIAL APPLICABILITY

[0117] According to the present invention, a stress sensor having highsensitivity can be provided. In addition, in a stress sensor in whichresistor elements composed of resistors provided with trimming groovesare used as strain gauges, in addition to the above, by suppressingplastic deformation of the resistors, a stress sensor capable ofmaintaining the accuracy of output resistance can be provided.

[0118] The stress sensor of the present invention can be suitably usedfor a pointing device for a personal computer, a multifunctional andmultidirectional switch for various electronic apparatuses, and thelike.

1. A stress sensor in which the direction and magnitude of a stress canbe grasped from variation in property of strain gauges caused by apressure applied thereto and the removal thereof resulting from theapplication of the stress.
 2. A stress sensor comprising: strain gaugesdisposed on a surface of a substrate; and a post disposed on one ofsurfaces of the substrate, in which the direction and magnitude of astress applied to the post can be grasped from variation in property ofthe strain gauges resulting from the application of the stress, whereinthe strain gauges are disposed on and the post is bonded to orintegrated with the same surface of the substrate.
 3. A stress sensorcomprising: strain gauges disposed on a surface of a substrate; and apost bonded to one of surfaces of the substrate, in which the directionand magnitude of a stress applied to the post can be grasped fromvariation in property of the strain gauges resulting from theapplication of the stress, wherein a post bottom surface and a part orthe entire area of each of the strain gauges overlap each other withoutthe substrate provided therebetween.
 4. A stress sensor in which thedirection and magnitude of a stress applied to a post can be graspedfrom variation in resistance of resistor elements, which are notprovided with trimming grooves, caused by stimulation applied theretoresulting from the application of the stress, wherein the stimulation isprimarily applied to resistor regions in which a current density ishigh.
 5. A stress sensor in which the direction and magnitude of astress applied to a post can be grasped from variation in resistance ofresistor elements caused by stimulation applied thereto resulting fromthe application of the stress, wherein the resistor elements areprovided with trimming grooves, the stimulation is primarily applied toresistor regions in which a current density is high, and the stimulationapplied to the resistor elements does not substantially open and closethe trimming grooves.
 6. A stress sensor comprising: resistor elementsdisposed on a surface of a substrate; and a post bonded to one ofsurfaces of the substrate, in which the direction and magnitude of astress applied to the post can be grasped from variation in resistanceof the resistor elements resulting from the application of the stress,wherein a post bottom surface and a part or the entire area of each ofthe resistor elements overlap each other without the substrate providedtherebetween, and the stimulation is primarily applied to resistorregions in which a current density is high.
 7. A stress sensor accordingto one of claims 4 to 6, wherein the resistor regions in which a currentdensity is high are formed of resistor regions in which a current passis narrowed.
 8. A stress sensor according to claim 5 or 6, wherein theresistor regions in which a current density is high are formed ofresistor regions in which a current pass is narrowed, and the resistorregions in which a current pass is narrowed are formed by trimminggrooves for adjusting resistance.
 9. A stress sensor according to one ofclaims 5, 7, and 8, wherein the trimming grooves are present onlyoutside or inside an outline of a post bottom surface.
 10. A stresssensor according to claim 8 or 9, wherein the stimulation applied to theresistor elements does not substantially open and close the trimminggrooves.
 11. A stress sensor according to one of claims 5, and 8 to 10,wherein the stimulation applied to the resistor elements provided withtrimming grooves, used as strain gauges, is caused by elongation orcontraction of the resistor elements, and the trimming grooves areprimarily formed substantially parallel to the direction of theelongation or the contraction.
 12. A stress sensor according to one ofclaims 1 to 11, wherein a post bottom portion has at least oneprojecting portion, and by the stress applied to a post, the projectingportion of the post bottom portion primarily stimulates the straingauges or resistor elements.
 13. A stress sensor according to claim 12,wherein the outer shape of a post bottom surface is a polygon, andindividual angular portions of the polygon serve as the projectingportions.
 14. A stress sensor according to claim 12 or 13, wherein theouter shape of a substrate is a polygon having at least one pair ofsides which are parallel to each other, a top portion of the post is apolygonal pole having at least one pair of side surfaces which areparallel to each other, and said pair of sides and said pair of sidesurfaces are parallel to each other.
 15. A stress sensor according toone of claims 12 to 14, wherein the projecting portion has a roundedshape.
 16. A stress sensor according to one of claims 1 to 15, whereinresistor elements are provided with a protection coating composed of amaterial softer than that for a substrate.
 17. A stress sensor accordingto one of claims 1 to 16, wherein resistor elements used as the straingauges are disposed at four locations on two lines, whichperpendicularly intersect each other at a center of a sensor effectiveregion on a surface of a substrate and which are along a surfacethereof, the four locations being at substantially the same distancefrom the center, and a post is bonded or integrated so that the centerof the sensor effective region on the surface of the substratesubstantially coincides with the center of a post bottom surface.
 18. Astress sensor according to one of claims 1 to 17, wherein a substrate isprimarily composed of a resin-based material, a metal covered with anon-conductive material on the surface thereof, or a ceramic.
 19. Astress sensor according to one of claims 1 to 18, wherein a post isprimarily composed of a metal, a ceramic, a resin, or a fiber-reinforcedresin.